Last weekend I had the opportunity to do some dyno runs on the mighty IV.
Background:
-Lemans IV motor
-Stock bore and pistons (949cc displacement)
-B-10 (stock) cam
-Mistral exhaust system with crossover under the transmission
-Compression increased to around 10.75:1
-Mildly ported heads. Valves were sunk in the valve seats somewhat to accommodate the Megacycle 620x8 cam that was previously installed. The valve areas were then de-shrouded to reduce flow restriction.
-Valve job performed 1000 miles previous to the dyno runs
-PHM 40 carbs with K&N filters.
-Jetting:
K19 needle- bottom clip
265 atomizer
130 main
62 idle
50/3 slides
Up to this point, I’d built the motor up and done a bunch of jetting iterations using the old fashioned “butt dyno” method. I wanted to validate my jetting using the dyno’s Lambda sensor.
The first set of runs was performed at the portable L&L dyno at Willow Springs track. I had an opportunity to do runs on their dyno the day before I had planned to take the Guzzi to Ken Zeller’s Evoluzione Cyclesports (www.evoluzione.net). All of the runs in this report were essentially done at wide open throttle, so they’re a poor indication of any part-throttle drivability issues (which is what Ken kept pounding into my head…see, Ken- it finally stuck. :-) ).
The lower end of the graph shows a torque dip between 2500 and 3500 RPM. I believe that this is actually an aberration, as this was around the point where the dyno tech whacked the throttle open. The resulting vacuum loss leaned the mixture out. All this indicates to me is that the bike can use a little twiddling on the accelerator pumps to lessen this effect. I had previously decreased the accelerator pump’s volume adjustment, thinking that they were over-richening the mixture in a transient sense. Going from full closed to full open throttle rarely occurs on the street, anyway- on the street, it’s all about throttle control.
For this run, I used a 130 main jet (also see my Lemans IV jetting article). Note that I had previously started with a 145(!!!!), which is stock for Euro jetting. At this point, WOT is still rich (~12:1). I later dropped mains down to128 (as lean as I had at the time) and got the A/F ratio to about 12.5:1. A 125 main will probably be where I end up.
Also note that I lost ~10 ft lbf (20%) of torque from 5500 to 7500 rpm. I’m going to concentrate on this part of the operating range in the future, as that 20% loss really makes the bike feel as if it’s run out of steam at high RPM’s. Looking at other Guzzi dyno runs, on guzzipower, and Doug Lofgren’s site, this looks to be a pretty typical torque characteristic of the Guzzi engines. If you look at some of Doug’s V11 sport testing, http://manleycycle.com/Tech/mgv11ex.htm, he was able to tune to keep torque from dropping off above 5500 rpm by playing with exhaust system components. Note that this is often at the expense of a flat spot in the midrange: tradeoffs, tradeoffs…. At this point, I was thinking that the addition of an exhaust crossover between the two header pipes above the alternator (a la the early and late Lemans factory pipes and the Bub systems) might decrease this top end torque loss. As later runs will show, there are torque benefits to be gained by paying attention to the induction as well.
OK, so the next day, I took the IV to Ken Zeller’s place for my scheduled dyno runs. The results are shown below.
<click to enlarge>
run 007: fully warmed bike with 128 main jet
run 010: removal of K&N filter from carb intake
Also note that while peak torque is within 2 lbf ft of L&L’s measurements, peak power is off by as much as 5 hp. I wonder how often either of these dynos are calibrated.
For figure 3, I leaned the main circuit with the 128 main jets that I had handy (I didn’t have anything leaner with me. damn... ). This brought A/F ratio to around 12.5:1. This is shown in run 007. After this, I pulled the dinky (and dirty) K&N filters off and re-ran the bike. Run 010 is the result. Note that the gains are mostly above 6500 RPM, where the serious torque dip occurred. At 7500 RPM, I’m making 4 more HP and 3 more ft lbf with the filters off. Ok. I’m convinced. The dirty filters are restrictive. Time for some higher flow filters…unfortunately, I’m limited by space constraints on the IV. We’ll see what I can come up with. Another note: A/F ratio was about 13:1 when I removed filters, so that also contributed to the better numbers. Filters on the street are a must. I’m simply not as cool as the Mike Acebos of the world who run open stacks on their Guzzis. The top end could probably benefit from some decreased minor flow losses (smooth velocity stacks instead of an abrupt carb opening) and possibly some airbox volume, so perhaps some intake runners with filters will liven up the bike’s top end. What have we learned: dirty filters can affect your jetting and your top end power, to the tune of a few horsepower.
I wanted to dyno the bike with an old Bub exhaust system with the front crossover pipe above the alternator, but I forgot to bring the header clamps with me to Evoluzione. My jury rigging the header pipes to the exhaust ports seemed to make Ken a little nervous, so exhaust permutations will have to wait until next time.
It’s been 1500 miles since I rebuilt my IV engine (after a torched rod bearing), and I’m getting antsy to build the motor a bit more. I don’t want the terrible low RPM running that came with the long duration high lift Megacycle 620x8. I’ll probably just add more compression, as I have clearance to spare and I’ve never heard the thing ping. I’d be perfectly happy with 5 more lbf-ft of torque across the RPM range, and a torque curve that doesn’t drop off as much after 6500 RPM. We’ll see where this all leads…
Guzzi Power (65 horses worth, to be exact)
Ed Milich
